Tasks of Technical Inspection
The main task of technical inspection is to promptly detect defects in the inspected objects and to prevent the production of castings that do not meet the specified requirements. According to the stage of production in which the inspection takes place, inspections can be divided into raw material inspection, process inspection, and acceptance inspection.
1. Raw Material Inspection refers to the inspection of raw materials supplied by other enterprises or departments.
2. Process Inspection refers to the inspection carried out during the casting production process, after completing one or several operations.
3. Acceptance Inspection is the inspection performed after all processes are completed, to determine whether the finished casting meets the requirements for delivery and use.
According to the quantity of inspected products, inspections can generally be divided into 100% inspection and sampling inspection.
Control of Casting Dimensional Accuracy
1. Factors Affecting Casting Dimensional Accuracy
In sand casting, the final geometry and dimensions of a casting mainly depend on two aspects:
· The geometry and size of the mold cavity, which are related to the dimensions and accuracy of the pattern, core box, and other tooling, as well as to the displacement of mold walls and deformation of cores before pouring.
· The expansion and contraction of the casting during solidification and cooling, which depend on the physical properties of the alloy, the displacement of mold walls during expansion, and the degree of restraint during contraction.
The factors affecting the dimensional accuracy of castings include:
• Pattern dimensional tolerance
• Pattern positioning tolerance on the pattern plate
• Core box dimensional tolerance
• Core deformation tolerance during core making
• Core positioning tolerance during assembly
• Casting shrinkage tolerance
Since these factors are complex, it is difficult to accurately predict casting dimensional accuracy in advance. Only by strictly controlling these influencing factors and minimizing their variations can the dimensional fluctuation of castings be reduced and the overall dimensional accuracy improved.
2. Measures to Improve Casting Dimensional Accuracy
(1) Design the correct working dimensions for patterns and core boxes
When the measured average size of the casting differs from the drawing, the working dimensions of the pattern and core box should be adjusted so that the average casting dimension meets design requirements. In batch or mass production, it is common to produce a trial batch of castings, measure their actual average dimensions, calculate the actual shrinkage rate, and then determine the appropriate working dimensions for the pattern and core box.
(2) Strictly control the manufacturing and assembly tolerances of tooling
The dimensional tolerance of patterns and core boxes directly affects the casting tolerance. Since these tolerances are formed during machining and are relatively easy to control, strict requirements should be imposed.
(3) Ensure accurate core dimensions and positioning
Core dimensional accuracy depends primarily on the core-making method. For the same method, it depends on the accuracy of the core box, displacement of the core wall during removal, and deformation before and during pouring. Cores with low green strength are easily deformed, resulting in poor dimensional control. Therefore, for castings with high accuracy requirements, hot box, shell, or self-hardening sand cores are preferred. The clearance between core prints significantly affects core positioning accuracy. After core assembly, inspection templates are typically used to verify and adjust the core position.
(4) Strictly control mold wall displacement
The tighter the mold is compacted, the less displacement occurs. If the pouring height is too great, the metal head pressure increases, which, combined with insufficient weight or improper fastening of the mold, can lead to excessive mold wall movement. Therefore, the related process parameters must be strictly controlled to minimize mold wall displacement and ensure dimensional accuracy.
Casting Surface Roughness
Surface roughness represents the degree of unevenness of an object’ s surface. Casting surface roughness refers to the micro-geometric features of the casting surface, consisting of small spacing and peaks and valleys. It generally depends on the casting material, casting method, and cleaning method (such as shot blasting, sand blasting, tumbling, etc.). Different areas of the same casting may have different surface roughness requirements.
Many factors influence casting surface roughness, such as the choice of casting method, mold surface roughness, chemical composition and pouring temperature of molten metal, the interaction between the molten metal and the mold surface, and the cleaning process of the casting.
(1) Choice of Casting Method
Casting methods include sand casting and special casting methods (such as metal mold casting, die casting, and investment casting).
Generally, special casting methods produce castings with lower surface roughness values.
The range of surface roughness values achievable by various casting methods
|
Casting method |
Surface roughness value Ra/µm |
|
Sand casting |
50~400 |
|
Die casting |
0.4~50 |
|
Investment casting |
0.8~12.5 |
|
Low pressure casting |
0.8~100 |
|
Permanent mold casting |
0.8~100 |
(2) Mold Surface Roughness
The surface roughness of the mold directly affects that of the casting. In sand casting, the grain size of the sand is the main factor determining mold surface roughness. Coarser sand results in a higher roughness value. For different castings, the grain size and distribution of the base sand should be chosen appropriately. Optimizing sand grading is the most effective and direct way to improve surface quality. Using finer sand or mixed sand with a well-distributed particle size reduces the gaps between grains, lowers mold surface roughness, and thus improves the casting surface finish.
(3) Molten Metal and Mold Interaction
When molten metal penetrates the gaps between sand grains on the mold surface, mechanical sand adhesion occurs, which increases surface roughness. The degree of metal penetration depends on factors such as the surface tension and pressure of the molten metal, as well as the gaps between sand grains on the mold’ s working surface.
Post time: Oct-10-2025